The invention relates to a method of, and apparatus for, setting registration, with detection of the entry of a printing substrate into a printing machine having a number of color printing units, and calculation of the beginning of digital production of color separations to achieve an in-register position of the color separation prints on a printing substrate.
Printing colored illustrations, in particular color images, is carried out by a number of color separations being printed over one another. These are generally the colors yellow, magenta and cyan as well as black. If required, special colors are added. By overprinting these colors, all color combinations can be achieved, the quality of the prints depending significantly on the in-register overprinting of the color separations. In the case of digital printing processes, for example electrostatic printing processes, the maintenance of the register of the overprint is achieved by the image production devices being controlled in such a way that the color separations meet one another in-register when they are transferred to a printing substrate. This has to be set and, if necessary, readjusted during printing. Since the color separations are produced anew for each individual printed image, a setting or correction after each printing operation is possible.
U.S. Pat. No. 5,678,128 discloses a method and apparatus of the type mentioned at the beginning in which a sensor for detecting the entry of a printing substrate is arranged, and control is provided for the start of printing in the printing units in order to achieve maintenance of register. Since the sensor for detecting the printing substrates is located within the machine, there remains only a very short time in which to be able to perform the calculation before the first color separation has to be produced in order to be transferred to the printing substrate. On the one hand, this requires a high computer capacity and, on the other hand. it limits the speed of the printing operation. If coincidence of register is to be achieved, not only with respect to the image starts but also with respect to a number of or many defined areas on the color separations, then the necessary computer processing requirement is increased considerably. However, this is not possible in the short time available. Therefore, in the prior art, the possibilities of achieving high maintenance of register of the color separations are also limited.
The invention is therefore based on the object of developing a method and apparatus such that more time is available for calculating the setting values for the registration setting.
With respect to the method according to the invention, the object is achieved by the printing substrate being detected before it enters the printing machine, and, following this detection, the relative positions of the color separations being calculated, and precise detection of the leading edge of the printing substrate being performed as it enters the printing machine and made the basis for determining the starts of the production of the color separations, their relative positions having been precalculated.
With respect to the apparatus, the object is achieved in that a sensor arranged upstream of the printing machine detects the printing substrate before it enters the printing machine and gives a signal to the computer, whereupon the latter calculates the relative positions of the color separations, and in that the sensor arranged in the printing machine gives a signal to devices for determining the release of start signals for the production of the color separations, whose relative positions have been precalculated.
By the invention, therefore, the relatively high processing requirement on computing has been dealt with before a printing substrate enters the printing machine, and the values determined are kept ready in such a way that the allocation of the leading edge of the printing substrate which has entered the printing machine to the precalculated relative positions is all that is required. The allocation is carried out by releasing start signals for the production of the color separations in accordance with precalculated data. This is associated with a low processing requirement on computing, which can be managed in a short time. In this way, in spite of the high computing processing requirement as viewed overall, precise and rapid allocation of the printing substrate to the calculated values is possible. This makes it possible for the invention to achieve an accurate relative position of the color separations. By the invention, a higher print quality can thus be achieved, and the calculation of the register setting is no longer a bottleneck to increasing the printing speed.
The abovementioned advantages of the invention already show an effect if only a calculation of the relative position of the image starts of the color separations is carried out. However, the prolongation of the computing time according to the invention is particularly advantageous when the calculation of the relative positions of the color separations comprises both the image starts and the position of defined areas into which the color separations are subdivided. Here, these regions can be lines of image points in the color separations, or a defined number of lines of image points.
One refinement of the invention provides for the production of the electrostatic latent color separations in order to achieve coincidence of register to be calculated and made the basis for open-loop or closed-loop control. Another, particularly advantageous refinement provides for the positions of the production of the color separations to be coordinated in relation to their points of transfer to the printing substrate in order to achieve coincidence of register, and with the paths of the printing substrate and up to the points of transfer. The second embodiment has the advantage that errors which result from the time/position assignment, for example from a time/distance or a time/angle assignment, are not included in the calculation. In this way, more exact open-loop or closed-loop control is achieved, and therefore a higher maintenance of register and better printing quality.
The apparatus can be developed further in such a way that it is capable of implementing all the aforementioned method features. In this regard, the following refinements of the apparatus are advantageous:
Provision is expediently made for the computer to be loaded with data for the relative position of the carrier elements for the color separations and a substrate carrier and performs a calculation which, with respect to the relative positions of the color separations, gives the image starts and the position of the defined areas into which the color separations are subdivided, and controls the equipment for the production of the color separations on this basis. The data for the relative position of the carrier elements and a substrate carrier can be obtained in various ways. These may be distance/time data relating to the elements which carry images or substrates, or it is possible to provide position data which relate to these elements and are assigned to one another. It is also possible to input calibration tables with appropriate values from experience, which can be assigned to various positions of elements carrying images or substrates. Most expedient is a calibration table which is assigned to the carrier element with a surface for the production of the color separations, which is generally an image cylinder. For such an assignment, the angular positions of these elements can be used. The aforementioned data can also be determined by printing and detecting register marks.
For the last-mentioned purpose, it is proposed that the apparatus have at least one sensor for detecting a component of a color separation, and the computer is such that it coordinates the relative positions, detected in this way, of the individual color separations with one another in order to achieve coincidence of register and coordinates the relative position of the substrate carrier in order to position the print on the latter. Such defined components of a color separation may be the register marks but it is also possible to use other defined components, such as the limits of a picture.
An advantageous development of the apparatus provides for sensors for detecting angular positions of the elements that transfer color separations and the angular positions of the drive roller of the substrate carrier, the computer being such that it coordinates the angular positions of the elements that transfer color separations with one another in order to achieve coincidence of register, and coordinates the angular positions of the drive roller of the substrate carrier with the aforementioned angular positions. The aforementioned sensors can also be sensors for detecting paths, for example of register marks, or they may also be sensors which detect traveling times of elements carrying images or substrates. It is therefore possible for sensors for detecting the paths of the surfaces of the carrier elements, that is to say for example of the image cylinders, to be provided, and sensors can also be provided for detecting the paths of the surfaces of image transfer elements, for example of transfer cylinders. A sensor for detecting the path of the substrate carrier is also possible for the assignment of the printing substrates to the color separations. However, angular position transmitters are preferably provided as the sensors, and a calibration is carried out for the corresponding angular positions.